Release stimulation

Histamine is usually released by degranulation of the storing cell. This is triggered by a rise in intracellular calcium. Mechanisms which produce this rise include:

• complement components C3a and C5a binding to receptors on the cell surface
• binding of IgE to cell surface in association with antigen
• histamine-releasing factors from various leukocytes e.g. neutrophils
• platelet activation e.g. contact with collagen, antigen-antibody complexes
• physical agents e.g. cold or trauma
• the cytokine interleukin-1

Release is less likely if cyclic AMP concentration is elevated. Beta-adrenoceptor agonists may modulate histamine-producing cells in this way.

Once released, the effects of histamine are reduced by:

• diffusion away from the site
• histaminase enzymes
• acetylation